The present invention relates to terrain following and, more particularly, to real-time methods for designing a terrain-following flight profile.
A combat air vehicle, for example a bomber, must fly close to the ground in order to reach its target without being detected and intercepted by the enemy. The general problem of flying as close as possible to the ground along a given flight path is known as the “terrain-following” problem. Early terrain following systems used active on-board sensors to determine the aircraft's position and terrain clearance. Radiation emitted by such active sensors could be detected by the enemy. As early as 1981, Alson C. Woodward and Wayne M. Hoover (“Passive terrain following using stored map and global positioning system”, IEEE NAECON 0547-3578/81/0000-0079) recognized that an aircraft with an on-board GPS receiver could follow a pre-stored reference terrain profile while using the GPS receiver to measure its own position. The aircraft actually follows a pre-stored reference flight profile that corresponds to the terrain profile. The design of the flight profile is an optimization problem: within the constraints of the aircraft's performance limitations, how close can the aircraft fly to the ground while following the flight path? See, for example, Ping Lu and Bion L. Pierson, “Optimal aircraft terrain-following analysis and trajectory generation”, Journal of Guidance, Control and Dynamics vol. 18 no. 3, May-June 1995. Usually, this optimization problem is solved iteratively.
One problem with the use of a pre-stored reference flight profile is that the actual flight profile followed by the aircraft usually deviates from the reference flight profile because of navigational uncertainties. These navigational uncertainties could be accounted for by computing the reference flight profile with respect to an “inflated” reference terrain profile, as described below; but at the cost of obtaining a suboptimal flight profile. The flight profile would be suboptimal because inflation in advance of the reference terrain profile necessarily would have to be based on “worst case” navigational uncertainties. There is thus a widely recognized need for, and it would be highly advantageous to have, a method for real time computation of a reference flight profile that would account for actual navigational uncertainties and so be less suboptimal than a reference flight profile computed in advance.